THE THEORY OF IMMUNITY 135 



ingestive powers, but their capacity of being used as 

 " complement " or a combining body. If sufficient of 

 their lipoids or proteins, say, is used up, they die and 

 become pus : if not, they survive and destroy the bacteria 

 which were attracted to them, and probably redigest the 

 lipoid complement taken up. In any case, the entrance 

 of the bacterium or toxin into the so-called phagocyte 

 is probably pathological. They may recover or perish. 

 If they die it is because they are used up in yielding parts 

 of themselves to the hostile cell. Every cell envelope 

 has lipoid substances in it ; but the phagocytes that cannot 

 yield any more from the outside yield it from within and 

 are destroyed. Even if that is not the cause of their 

 death, they may die from toxins entering into chemical 

 or molecular union with the lipoids of the membrane, 

 which is rendered functionless. If these suggestions have 

 any foundation, the leucocytes generally have another 

 role in defence than that commonly stated. And we 

 may infer that the substrate " seeks " them and their 

 lipoids under the influence of the special catalyst, rather 

 than that they " seek " the substrate. If this is so, 

 opsonins do not exist, and the phenomena they are supposed 

 to explain represent the fact that the easiest reached 

 " complement " is found by the tropisms of the bacteria 

 and the polynuclears. The leucocytes are the cheapest 

 sacrifice the body can make. 



There seems plenty of evidence that enzymes and all 

 catalysts appear only when the organism is stimulated by 

 the particular substrate with which they deal. There are, 

 however, according to Bayliss, cases in which their par- 

 ticular substrate is wanting. For instance, it seems that 

 lactase is found in almonds, as adrenalin is found in the 

 skin of the toad. We say, then, that it is an accidental 



